A PFC circuit is typically adopted in a high-power supply source to improve a power factor of the supply source. For a PFC circuit with a structure of positive and negative bus bars, whether voltages of the positive and negative bus bars are balanced will influence a voltage stress on a switch tube in the structure of the PFC circuit, over-voltage protection strategy and other issues, and it is thus necessary to take measures to equalize the voltages of the positive and negative bus bars so that the voltages of the positive and negative bus bars are in an equalized condition.
A biased voltage loop is adopted in the PFC circuit to control the voltages of the positive and negative bus bars to be in an equalized condition, and the biased voltage loop adjusts charging energy of the positive and negative bus bars primarily by affecting the transmitted waves of Pulse Width Modulation (PWM) and further renders the voltages of the positive and negative bus bars in an equalized condition. Typically when the hardware design is reasonable and the loop design is appropriate in the PFC circuit, the biased voltage loop has a good effect of equalizing the voltages of the bus bars; but when the PFC circuit operates in an approximately uncontrolled rectification condition or is lightly loaded, the ability of the biased voltage loop to adjust and equalize the voltages of the bus bars will be weaken and fails to well control the equalization condition of the positive and negative bus bars, thus resulting in a serious bias voltage phenomenon. This is primarily because when the PFC circuit operates in an approximately uncontrolled rectification or lightly loaded condition, there is a small amount of transmitted waves of PFC circuit and even an intermittent wave transmission condition (waves are transmitted for a period of time and then ceased for a period of time), so that the biased voltage loop will be operative for a limited period of time and the ability to adjust and equalize the voltages of the bus bars will not be effectively enforced, thus unbalancing the voltages of the bus bars.